12 NEW YORK STATE MUSEUM 



The inference as to their origin is more readily apparent in the case 

 of the limestones and schistose types than in reference to the more 

 massive gneisses, for which the field evidence alone is seldom 

 determinative. 



Plutonic igneous masses invaded the region at different times 

 during the Precambric period. They have broken up the sedi- 

 mentary rocks into isolated areas, injected them with their materials 

 and blended with them along the contacts. Subsequent com- 

 pression has converted them into gneisses which are often hard to 

 distinguish from those of the sedimentary class. A later manifes- 

 tation of igneous activity led to the intrusion of dike rocks. 



While a great part of the crystallines can be differentiated into 

 the two classes of igneous and sedimentary derivatives, there are 

 considerable areas of gneisses whose origin has not been fully estab- 

 lished. Their relationships have been obscured by profound altera- 

 tion, leaving little evidence as to their original nature. It is in 

 connection with these rocks that the principal stratigraphical prob- 

 lems remain to be solved. 



According to the classification generally employed for Precambric 

 rocks, the sedimentary gneisses fall within the Grenville series. 

 If any rocks exist in the region which antedate the oldest sediments 

 of that group, they are probably to be found among the gneisses 

 previously mentioned. 



Sedimentary, or Grenville, series. So far as known, the sedimentary 

 derivatives are the oldest rocks in the Adirondacks. They possess 

 much similarity in their development and individual constitution 

 to the Grenville series of Canada, with which they are now generally 

 correlated as the nearest equivalent in age. They are believed to 

 be ancient water deposits and if so must have been laid down upon 

 some floor of still older rocks that have not yet been definitely 

 recognized. Little is known as to the thickness of the series, though 

 from the facts of their distribution it is concluded that they must 

 have been originally very thick. The variation in composition, 

 from original calcareous and magnesian deposits to shales and sand- 

 stones and probably coarse conglomerates, as well, is such that it 

 can be explained only by wide-reaching changes in the processes 

 of accumulation that require long lapses of time. Neither the base 

 nor the top of the series has been identified. 



Limestone. The limestones have the crystalline texture of marbles , 

 they range from nearly pure lime carbonates to magnesian lime- 

 stones and dolomites. They are always impregnated by foreign 

 minerals that have been formed out of the carbonates and the 



